As disclosed in Patent Literature 1, a particle size distribution measuring apparatus of this kind is intended to detect diffracted/scattered light by a plurality of photodetectors and calculate a particle size distribution based on the following Expression (1) using a light intensity pattern vector obtained by a value of a light intensity signal outputted from each of the photodetectors.[Equation 1]s=Kq  (1)
Here, s is a vector representing a light intensity pattern at every angle of diffracted/scattered light obtained from a value of a light intensity signal outputted from each of the photodetectors, q is a vector representing a particle size distribution of the particles to be measured, and K is a coefficient matrix for converting the particle size distribution vector to the light intensity pattern vector.